The present invention relates to a radio communication apparatus and, more particularly, to an improved baseband signal processing circuit including an amplifier and a limiter.
A conventional baseband signal processing circuit has an amplifier for level adjustment, a limiter, a splatter filter, and a modulator. The amplifier amplifies an input baseband signal and feeds the resulting signal to the limiter. The limiter limits the amplitude of the amplified baseband signal to prevent the frequency band from broadening in the event of transmission or, in the case of FM, limits the deviation. On receiving the output of the limiter, the splatter filter removes the frequency band for broadband from the limiter output. The output of the splatter filter is applied to the modulator.
In the conventional circuit described above, when the input signal to the limiter has an extremely great amplitude (infinite), the output of the limiter has a rectangular waveform whose amplitude has been limited at a predetermined level. The splatter filter removes, among the frequency components of the rectangular wave, high frequency components. However, when only the fundamental frequency component of the frequency of the rectangular wave is passed through the splatter filter, the output of the filter has a greater amplitude than the rectangular wave. Specifically, the amplitude is 4/.pi. times greater than the amplitude of the rectangular wave. More specifically, at the stage following the splatter filter, the amplitude is 4/.pi. times greater in the frequency range in which only the fundamental frequency component passes than in the frequency range in which all the frequency components pass, despite that the limiter limits the amplitude. Regarding an FM (Frequency Modulation) transmitter, the limiter limits the maximum frequency transition. However, the maximum frequency transition has to be further reduced in the frequency range in which the output amplitude of the splatter filter is 1, since it should be limited with 4/.pi. used as a reference. This reduces the difference between the standard and maximum frequency transitions and, therefore, is apt to cause distortion to occur at signal levels exceeding the standard frequency transition.